This specification relates to selecting content items based on presentation context.
The Internet has enabled access to a wide variety of resources, e.g., video and/or audio files, web pages for particular subjects, news articles, etc. Access to these resources has likewise enabled opportunities to provide additional content with the resources. For example, a user device may request a resource, such as a search results page from a search engine, by submitting a search query to a search engine. The search results page includes instructions that cause the user device to request content items for presentation with the search results page. Content items of particular interest to a user can be identified by a content management system in response to the user request. The request can include information the content management system can use to identify and, optionally, rank the content items so that the content items are presented to the user according to the rank in content item slots on the search results page.